This invention relates to radio broadcasting, and more particularly, to modulation formats for use in AM In-Band-On-Channel (IBOC) Digital Audio Broadcasting (DAB), and broadcasting systems utilizing such modulation formats.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. AM IBOC DAB can be transmitted in a hybrid format where it coexists with the AM signal, or it can be transmitted in an all-digital format where the removal of the analog signal enables improved digital coverage with reduced interference. Initially the hybrid format would be adopted allowing existing receivers to continue to receive the AM signal while allowing new IBOC receivers to decode the DAB signal. In the future, when IBOC receivers are abundant, a broadcaster may elect to transmit the all-digital format. The DAB signal of the all-digital format is even more robust than the hybrid DAB signal because of allowed increased power of the former with a digital time diversity backup channel. IBOC requires no new spectral allocations because each DAB signal is simultaneously transmitted within the same spectral mask of an existing AM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
U.S. Pat. No. 5,588,022 discloses a hybrid AM IBOC broadcasting method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel that includes the steps of: broadcasting an amplitude modulated radio frequency signal having a first frequency spectrum, wherein the amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal; and simultaneously broadcasting a plurality of digitally modulated carrier signals within a bandwidth which encompasses the first frequency spectrum, each of the digitally modulated carrier signals being modulated by a portion of a digital program signal, wherein a first group of the digitally modulated carrier signals lying within the first frequency spectrum are modulated in-quadrature with the first carrier signal, and wherein second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. Recent developments in AM IBOC DAB systems are discussed generally in "Improved IBOC DAB Technology for AM and FM Broadcasting," by B. Kroeger, and A. J. Vigil, presented at the 1996 National Association of Broadcasters SBE Conference, Los Angeles, Calif., November, 1996.
As audio coding algorithms continue to improve, acceptable audio quality can be obtained at lower data rates and with less error protection due to embedded techniques than were envisioned for use in the method of U.S. Pat. No. 5,588,022. This invention seeks to provide methods for AM IBOC hybrid and all-digital broadcasting which take advantage of the characteristics of recently developed coding algorithms and addresses the typical interference patterns of AM broadcasting channels.